In fiction, Dark Lord (or Evil Overlord) is often used to refer to a powerful villain or antagonist with evil henchmen. In particular, it is used as a moniker in universes where it is thought that pronouncing the villain's real name will bring bad luck or represents a bad omen. Such a villain usually seeks to rule or destroy the universe around them.

In fantasy novels, Dark Lords have become something of a cliché stemming from the success of J. R. R. Tolkien's The Lord of the Rings, in which the main antagonist, Sauron, is often referred to as the "Dark Lord". On occasion, the people of Gondor in Middle-earth refer to Sauron as "The Enemy" or "The Nameless Enemy" despite knowing his real name; arguably starting the practice of avoiding pronouncing a Dark Lord's actual name. In Tolkien's legendarium, Sauron is the second Dark Lord; he was the lieutenant of the first Dark Lord, Morgoth (also a title or epithet meaning "dark enemy"), until the latter's defeat. Morgoth's original name was Melkor (he who arises in might).[1] Following the example of Sauron, Dark Lords in fantasy are always depicted as immensely powerful and implacably evil creatures with a great desire for power. One example of a powerful Dark Lord in the world of literature is the Dark Wizard Lord Voldemort from J. K. Rowling's Harry Potter novels, earning his place as a Dark Lord having surpassed the magical abilities of any other dark wizard before him, thus he is considered not a Dark Wizard but the first and only Dark Lord within that universe, being so powerful and evil that his enemies even dreaded to speak his name. Dark Lords have a negative effect in their universe, throwing them into ruin and despair. Sauron, for example, turned Mordor into a "wasteland where the very air saps one's will". He planned to do the same to all of Middle-earth. Dark Lords have mostly been male, with few exceptions such as the White Witch from The Chronicles of Narnia, who casts the world into an eternal winter but never Christmas.

Dark Lord characters do not often engage in direct conflict with protagonists. They are dark gods, demons or rulers of lands who exist in other dimensions, and/or maintain a dark, inaccessible fortress. They rely on a vast network of minions, often with an extremely hierarchical structure. In Star Wars, "Dark Lord" is a rank achieved by those who become Sith Lords as in the "Dark Lord of the Sith". The most recognized Dark Lords of Star Wars are Darth Vader and Darth Sidious.

The frequency in which the Dark Lord cliche occurs spawned the Evil Overlord List, a website satirizing the mistakes of Dark Lords and major villains. Frequently, antagonists in fiction will display numerous Dark Lord mannerisms while belonging to another genre of fictional villain, and some pertain to more than one genre. An example is Davros from Doctor Who, whose position as creator of the Daleks and later ruler of their empire marks him both as a Dark Lord and a mad scientist.

In comics, villains are usually considered Dark Lords either by the format of the story in which the villain appears or because of the villain’s modus operandi. For example, Ming the Merciless, Thanos and Darkseid are alien despots and could fall under the category of alien invaders. However, they exist within stories of such operatic nature, with elements of swashbuckling adventure and mythological analogy, that they are considered specifically to be Dark Lords. Alternatively, comic book villains The Kingpin and the 1990-era Lex Luthor could be considered modern-day versions of a Dark Lord, but more closely fall under the categories of a crime lord or a mad scientist, respectively. This is mostly due to these characters traditionally seeking a public identity as a businessman or a philanthropist, while keeping their criminal activities secret. This is at odds with one of the hallmarks of a Dark Lord, which is that they act from or deliberately seek out a position of legal authority, albeit often self-appointed, and even their most nefarious deeds are often performed publicly.

A recent example can be found on the television series Once Upon a Time, where the title "The Dark One" identifies someone with prolific magical powers whose life and powers are bound to a dagger bearing their name as an inscription. Whoever possesses the dagger can control the Dark One, but if they kill the Dark One with it, they become the new Dark One themselves. In this series, the first Dark One was Nimue the lover of Merlin. For 1000 years the Dark Ones have terrorized the people of the enchanted forest leading up to the current Dark One, Rumplestiltskin/Mr. Gold.

Many Fantasy games, as well as some of the SciFi genre, feature a Dark Lord who rules over one faction in the game. While this character is often the antagonist and final boss, some RPGs, such as Dark Souls, allows the player to earn this title for themselves. As this would often make the player character be in-charge of the game's hordes of enemies the moniker is usually granted at one of multiple endings. In order to make sequel games, this is often considered the non-canon ending, as in Star Wars: The Force Unleashed, but some games, like Blood Omen: Legacy of Kain, use this ending to turn the powered up anti-hero into the series' new antagonist!